52226-mt----surge Phenomena & Insulation Coordination

NR

Code No: 52226/MT

M.Tech. – II Semester Regular Examinations, September, 2008 SURGE PHENOMENA & INSULATION COORDINATION (Power Systems ( High Voltage) ) Time: 3hours

Max. Marks:60 Answer any FIVE questions All questions carry equal marks ---

1.a) b)

Derive the expression for voltage and current waves on transmissions lines and obtain the surge impedances of the lines. An overhead line has the following parameters: Resistance= 74 × 10−6 ohms/m Inductance= 1.212 × 10−6 H / m Capacitance= 9.577 × 10−12 F/m Leakage inductance=negligible Assuming the voltage and current surge waves in the line remain similar and in the ratio of surge impedance, calculate the distance that the surge must travel to attenuate to half value.

2.a) b)

3.a) b) 4.a) b)

What is Bewely’s Lattice diagram? Explain its importance in estimating the over voltages due to successive reflections. A line of surge impedance 400 ohms is terminated by a capacitance of 1Χ10-12 F. Set up the Bewely’s lattice diagram and calculate the voltage built up on the capacitance after 5 reflections and after a long duration. The line is open ended and surge is a step voltage of 100kV Give the mathematical models of lightning discharge and explain them. Describe the mechanism of lightning stroke with neat sketches. What are the causes for switching and power frequency over voltages? How are they controlled in a power system? Discuss the origin and characteristics of over voltages due the to switching surges in a power system. Contd…2

Code No: 52226/MT 5.a) b) 6.a)

b) 7.a) b) 8.a) b)

::2::

Explain the principle of resistance switching. Why it is necessary in air blast circuit breakers. Write a short note on the auto re-closure in circuit breakers. A voltage wave of 2500kV is traveling on a line of surge impedance 275 ohms. The attester is connected to the line has a protective level 1500 kV. Calculate: i) The current in the wave, ii) The current through the arrester, and iii) The arrester resistance at this current. Describe the various, methods protecting the rotating machines against the switching surges. Discuss the breakdown models of a long air gap with non-uniform fields. Explain clearly about the CFO and withstand voltage of long air gaps. Briefly explain the insulation coordination based on lightning. What is BIL? Discuss its role in insulation coordination.

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